Synonyms

Mustela rixosa

Basis for Listing

The least weasel's range in North America stretches from Alaska southeast through Canada and into the northcentral and northeastern United States, probably including the entire state of Minnesota (Hazard 1982). However, most records of this species in Minnesota come from the northwestern portion of the state. Once considered secure in the state, only one least weasel has been recorded in Minnesota since 1967 despite extensive survey work in suitable habitats. Competition from ermines (Mustela erminea), a related and more common weasel, may be a contributing factor to the rarity of least weasels. Other factors which may pose a threat to this species include habitat and prey loss, poisoning, and predation. The least weasel was listed as a special concern species in Minnesota in 1996.

Description

The least weasel is the smallest of all weasels, averaging 157-190 mm (6.2-7.5 in.) in total length and weighing 40-56 g (1.4-2.0 oz.) (Hazard 1982). It has a long body and neck with short limbs, a short tail, and a narrow, flattened head. In the summer, least weasels are brown on the back and whitish with occasional brown flecks underneath. In winter, they are entirely white in northern latitudes, where snow is common, but remain brown in southern latitudes (Sheffield and King 1994). The least weasel resembles other weasel species, but it is smaller and has a proportionately shorter tail than the ermine and the long-tailed weasel (Mustela frenata). Its tail is less than 25% of the length of its head and body and lacks the black tip characteristic of ermines and long-tailed weasels (Sheffield and King 1994). The pelage of least weasels will fluoresce under UV light, while that of ermines and long-tailed weasels will not (Hazard 1982; Svendsen 1999).

Habitat

Least weasels are found in a variety of habitats. While their habitat selection is generally a function of small rodent distribution, they seem to prefer meadows, grasslands, and marshy and shrubby habitats (Jones and Birney 1988). They are generally not found in dense forests or sandy deserts (Sheffield and King 1994).

Biology / Life History

Least weasels are specialized small mammal predators, eating primarily mice and voles. When small rodents become scarce, bird eggs and nestlings, moles, shrews, young rabbits, squirrels, rats, lizards, salamanders, frogs, fish, and insects may be taken. Least weasels are active during the day and night, patrolling a regular hunting route. They move rapidly searching every hole, tunnel, or burrow they can in search of mice. Their senses of sight, hearing, and smell are sharp and each contributes to finding prey. Typically, an individual will need to eat 40%-60% of its body weight each day in order to survive. Least weasels are preyed upon by most predators larger than themselves including hawks, owls, other weasels, foxes, cats, and snakes.

Male and female least weasels each defend a territory from others of the same sex. Territories of up to 24.3 ha (60 ac.) are established using a pungent odor exuded from anal glands. The nests of voles or mice are overtaken for use in raising young. Males and females come together only for a brief period during the breeding season which may occur year round, if prey is abundant, but typically occurs in spring and late summer. Females may produce 2 litters per year. An average litter in North America is 4-5 young, however, in artic regions as many as 15 young may be born to a litter when rodents are plentiful. Gestation lasts 34-37 days and the young are born altricial. They are weaned after 6-8 weeks, but will stay with their mothers until they reach maturity. The family group breaks up after 9-14 weeks. Females born in spring are sexually mature after 3 months and may produce young themselves in their first year. Summer or fall-born females will not become sexually mature until the following spring. Average annual mortality for least weasels is 75%-90% and the average life span is less than one year (Sheffield and King 1994; Svendson 1999).

Conservation / Management

Likely threats to least weasels include historical human trapping, habitat and prey loss, poisoning, competition from other species, and predation. While this species' pelt is so small that it is generally of little use, it was trapped historically. Weasels have often been blamed for game bird deaths, and have been a focus of past predator control. While this species may take some chicks, it is not likely to have an effect on bird populations.

Agricultural changes have led to the loss or reduction of habitat for this species and its main prey items. In years with low rodent populations, many weasels perish and local populations may experience extinctions. If sufficient habitat surrounds the extirpated population, other least weasels may recolonize the areas when food becomes abundant. There is also evidence that rodenticides may have an effect on weasels, as they eat poisoned rodents (McDonald et al. 1998). Competition with the much more common ermine might also be a contributing factor in this species' observed decline in Minnesota.

Conservation Efforts in Minnesota

The Minnesota Biological Survey has targeted this species for many years, and has located only a single individual despite intense trapping activities. A sub-adult female was captured in a live trap in 2006 on a privately owned native prairie tract in Murray County. Prior to this capture, no specimens of this species had been collected in Minnesota since 1967. While it seems that least weasel populations have fluctuated historically in Minnesota (Swanson and Fryklund 1935), the low level of observations is troubling. More research is needed to determine the extent of least weasel distribution in Minnesota and the ecological requirements necessary to ensure this species' survival.

Svendsen, G. E. 1999. The Least Weasel (Mustela nivalis). Pages 173-174 in D. E. Wilson and S. Ruff, editors. The Smithsonian Book of North American Mammals. Smithsonian Institution Press in association with the American Society of Mammalogists, Washington.

Swanson, E., and P. O. Fryklund. 1935. The Least Weasel in Minnesota and its fluctuation in numbers. The American Midland Naturalist 16:120-126.